The present invention relates to a method for restoring a formation which has undergone an in situ leaching operation and more particularly relates to a method for restoring a previously leached formation so that oxidized mineral values in the formation will not contaminate ground waters after the leach operation has been completed.
In a typical in situ leach operation, wells are completed into a mineral-bearing formation (e.g. uranium) and a leach solution is flowed between wells to dissolve the uranium values into the leach solution. The leach solution is then produced to the surface to recover the uranium values. As is well known, uranium and other leachable minerals often occur in the formation in a reduced state and must be oxidized in order to render them soluble in the leach solution. To oxidize the minerals, an oxidant (e.g. oxygen, hydrogen peroxide, etc.) is flowed through the formation prior to or along with the leach solution.
Unfortunately, where the leached formation also contains ground waters and/or a water source that may have originally been suitable for surface use, the oxidized mineral values such as uranium and molybdenum remaining in the formation after a commercial recovery operation is completed pose a severe contamination threat to the formation waters. This is due to the fact that the oxidized values remaining in the formation will continue to dissolve into the formation water and will be produced therewith. If the amount of a particular mineral in the produced formation water exceeds the recognized safety level for that particular mineral, the formation must be treated after a leach operation to remove the threat of contamination from these oxidized minerals and to restore the purity of the formation water to substantially its original base line quality.